This video frame grab provided by Senate Television shows Sen. Rand Paul, R-Ky. speaking on the floor of the Senate on Capitol Hill in Washington, Wednesday, March 6, 2013. Senate Democrats pushed Wednesday for speedy confirmation of John Brennan's nomination to be CIA director but ran into a snag after a Paul began a lengthy speech over the legality of potential drone strikes on U.S. soil. But Paul stalled the chamber to start what he called a filibuster of Brennan's nomination. Paul's remarks were centered on what he said was the Obama administration's refusal to rule out the possibility of drone strikes inside the United States against American citizens. (AP Photo/Senate Television)

WASHINGTON (Reuters) - Kentucky Republican Rand Paul took the floor of the Senate on Wednesday and spoke for nearly 13 hours in an attempt to block the confirmation of John Brennan as President Barack Obama's next CIA director in a protest of the administration's policy of using drones in targeted killings.

Paul began shortly before noon in a rare old-fashioned filibuster - in which a senator speaks until he can continue no longer - yielding the floor shortly before 1 a.m. ETon Thursday.

"I've discovered that there are some limits to filibustering, and I am going to have to take care of one of those in a few minutes here," he said to laughter, after thanking his supporters, staff and U.S. Capitol employees.

Paul's action did not keep the Senate from voting on whether Brennan, Obama's counter-terrorism advisor, should lead the U.S. spy agency, but he delayed it until at least Thursday.

After Paul finished, Senator Richard Durbin, the number two Democrat in the Senate, entered motions to clear the way for the confirmation to move ahead.

There could still be a further wait for a final confirmation vote, because of Senate rules that require 60 votes to stop debate, followed by a waiting period before a final vote.

It was not clear whether Republicans would agree to waive those rules. The top Senate Republican, Mitch McConnell, who like Paul represents Kentucky, said during the filibuster that he would not back a motion to end debate and clear the way for a vote.

Brennan's confirmation, which requires only a simple majority of 51 votes in the full 100-member Senate, is expected when the vote finally takes place. The Democrats control 55 votes in the Senate and Brennan is also supported by many Republicans.

The filibuster was not a protest of Brennan's qualifications to lead the CIA, but a vehicle for Paul to draw attention to his demands for more information about the drone program.

Best known for his libertarian tendencies, Paul said he was protesting against a U.S. policy of using unmanned drone aircraft in foreign conflicts, as well as Attorney General Eric Holder's refusal to rule out any possibility of drone strikes against U.S. citizens on American soil.

"I will speak until I can no longer speak. I will speak as long as it takes, until the alarm is sounded from coast to coast that our Constitution is important, that your rights to trial by jury are precious, that no American should be killed by a drone on American soil without first being charged with a crime, without first being found to be guilty by a court," Paul said.

SEEKING ANSWERS ON DRONE POLICY

Twelve other Republicans, including Senators Mike Lee of Utah, Texas' Ted Cruz, Marco Rubio of Florida and Pat Toomey of Pennsylvania, who had just had dinner with Obama, took the floor to ask Paul questions during the filibuster, giving him a break.

McConnell's participation late in the evening signaled that Paul's cause had extended beyond the Republican Party's most conservative wing.

Liberal Democratic Senator Ron Wyden, who has expressed strong doubts about the drone program, also participated in the filibuster, despite his vote for Brennan in the Senate Intelligence committee.

Durbin also spoke, but used his time to question some of Paul's assertions and suggest that he give up and raise his concerns in a Senate hearing.

Paul was the first senator to embark on a traditional talking filibuster - of the sort immortalized in the 1939 movie "Mr. Smith Goes to Washington" - since the Senate voted early this year to make it more difficult to stop votes by using procedural tactics.

The last talking filibuster was by Bernie Sanders, an independent from Vermont, who spoke for more than 8-1/2 hours against a tax bill in 2010. That speech was turned into a book.

The longest filibuster on record was by the late Senator Strom Thurmond, who spoke for 24 hours and 18 minutes against the Civil Rights Act of 1957.

On Tuesday, the Senate Intelligence Committee voted by an overwhelming 12-3 majority to approve Brennan's nomination and send it to the full Senate for a final vote.

Some Republican critics of President Barack Obama's administration had threatened to try to delay the nomination until the White House discloses more information about its response to the attack on a U.S. diplomatic compound in Benghazi, Libya, on September 11 last year.

The Intelligence Committee put off a vote on Brennan last week because of squabbling among both Democratic and Republican committee members and the White House over congressional access to sensitive documents related to the Benghazi incident and the use of drones to attack suspected militants.